A spectrometer is an apparatus for measuring a spectrum of measurement light to be measured. There is known, as an example of the spectrometer, a Fourier transform spectrometer configured to obtain a spectrum of measurement light by measuring interfering light of the measurement light by an interferometer and by subjecting the measurement result to a Fourier transform.
In the Fourier transform spectrometer, an output of the interferometer is represented by a synthesized waveform, in which light of a plurality of wavelengths included in the measurement light is interfered by the interferometer as a lot. The output is called as an interferogram. A spectrum of measurement light is obtained by subjecting the interferogram to a Fourier transform. The interferogram has such a profile that one or more sharp peaks appear within a predetermined range, and that the output becomes substantially zero level within the remaining range. A center peak out of the one or more sharp peaks is called as a center burst.
In the Fourier transform spectrometer, if a spectrum of measurement light is obtained by subjecting an interferogram obtained by one-time measurement to a Fourier transform, normally, the S/N ratio is poor, and it is difficult to obtain a measurement result with intended precision. In view of the above, in the Fourier transform spectrometer, an interferogram is measured a plurality of times with respect to one measurement object, and the interferograms are integrated for generating an interferogram (hereinafter, called as an integrated interferogram) for use in obtaining a spectrum of measurement light. In the ordinary practice, the plurality of times of measurements are performed while continuously changing the optical path length of one of the two optical paths of the interferometer.
The technique of integrating interferograms is disclosed in patent literature 1 and in patent literature 2, for instance. The interferogram integrating device disclosed in patent literature 1 is an interferogram integrating device which integrates unit interferograms to be obtained by irradiating a measurement object with interfering light of one scan. The interferogram integrating device is provided with unit interferogram storing means which temporarily stores the unit interferograms, maximum position detecting means which detects a center burst position, based on unit interferogram data stored in the unit interferogram storing means, cutting means which cuts a predetermined amount of unit interferogram at both ends of the unit interferogram on a positional axis of the unit interferogram, with respect to the center burst position of unit interferogram detected by the maximum position detecting means as a reference so as to extract a cut interferogram, and integrating means which integrates a plurality of the cut interferograms successively obtained in correspondence to the unit interferograms.
Further, in patent literature 2, a measurement light interferogram generated by transmitting measurement light through a measurement object, and a reference light interferogram generated by allowing reference light to bypass the measurement object are synchronously measured. There is computed a phase difference at which the phase of a reference light interferogram in a present measurement period maximally coincides with the reference light interferogram stored in advance as a reference in a reference waveform storage. Then, an average of the measurement light interferogram and the reference light interferogram is obtained by performing synchronous addition with respect to the computed phase difference.
In the case of integrating interferograms as described above, it is necessary to sum up measurement data having the same optical path length difference as each other with respect to each of the interferograms. For the summation, measurement data within a range including a center burst is extracted from a plurality of measurement data (measurement data at the respective sampling points) obtained by one-time measurement, and then, measurement data having the same optical path length difference as each other is retrieved. Thereafter, the measurement data having the same optical path length difference as each other are summed up.
In the case of using a light reflecting mechanism for moving a reflection surface in a direction perpendicular to the reflection surface by resonant vibration in order to change the optical path length of the interferometer, for instance, intrusion of noise e.g. external vibration may vary the amplitude of the reflection surface resulting from an influence of the noise (such as external vibration). As a result, in the case where measurement data is extracted a plurality of times of measurements within a certain range, in some cases, a center burst may not be included in the extracted measurement data within the range. In such a case, it is impossible to perform positioning of the interferograms, and it is impossible to integrate the interferograms over the entirety of each of the optical path length differences.